PPP Configuration

PPP- WAN Protocol

Point-to-Point Protocol is a Layer 2 WAN protocol that can be configured on Cisco Routers.

– It is an Open Standard WAN protocol
– It is a Data-Link Layer protocol
– Can be used for synchronous serial i.e. ISDN or for asynchronous serial i.e. dial-up
– PPP supports authentication
– It also supports compression
– Supports error correction

PPP has 4 main components namely:

Physical : An International Standard for serial communication like ISDN, EIA/TIA 232, V.24 and V.35

LCP: Link Control Protocol: helps to establish, maintain and terminate a point-to-point connection. The link is brought down gracefully if not used. LCP will help to set up the link.

NCP: Network Control Protocol: is used to establish and have different Network Layer Protocols configured for transport across PPP link. Simultaneous use of multiple Network layer Protocols is possible using NCP. Different protocols can be run over PPP like IP, IPv6.

Features of PPP

Authentication: used for secure communication by the sending and receiving devices making the use of passwords for authentication. Two types of authentication are supported PAP and CHAP.

Compression: By compressing the payload or data before sending it we can get increased throughput. Stac and Predictor are two forms of compression supported by PPP.

Error Detection: A reliable and loop free data link layer can be ensured using Quality and Magic Numbers.

MultiLink: As its name specifies, it does help in make several physical paths look to be one logical path at Layer 3. The channels are bundled or trunked in order to combine bandwidth.

Callback: we can have PPP configured in such a way that it calls backs after authentication. If we have call back enabled a client can contact a server and authenticate.

3 Phase Session Establishment

On routers running PPP when the connections start, the three phases the link go through are:

Link establishment phase using LCP

Authentication ( this is optional)

Across the PPP links need to make sure that IP and other protocols can be sent ( NCP take care of this)

PPP Authentication Methods

In PPP two authentication methods are available:

PAP: Password Authentication Protocol

CHAP: Challenge Handshake Authentication Program

1. PAP: the passwords are sent as clear text and thus less secure. Also the PAP occurs when the link is initially established.

2. CHAP: this is used at the initial startup and at periodic intervals to know that the communication between routers is proper. After the link establishment phase a challenge is sent to the remote router by the local router. A value calculated using MD5 (one way hash function) is sent by the remote device. This hash value is matched by the local router to confirm it matches, if match does not occur then the link will be terminated.

By default on all Cisco routers HDLC will be configured.

The show controllers < interface> command will show if the router is DCE or DTE.

Router_A

Router_B

Configure IP address on the 2 routers

Router_A

Router_B

Now we will change the encapsulation from HDLC to PPP. We use the encapsulation ppp command.

Router_A

Router_B

Configure PPP authentication

Router_A

Router_B

Note: For PPP Authentication the passwords must be same.

Router_A

You can verify the PPP configuration by checking show interface < interfacename>